Anaerobic bacteria of the genus Bacteroides are inherently resistant to most commonly used antibiotics (penicillin, aminoglycosides). The lincosamide antibiotic clindamycin (Cc) is routinely used to control Bacteroides infections but resistant strains are being isolated more frequently. Studies on the genetic and molecular basis of Cc resistance in Bacteroides were performed. A Bacteroides ovatus clinical isolate transferred Cc resistance by a conjugation-like mechanism and was found to harbor two plasmid species, pBI136 (83 kilobases [kb]) and pBI106 (44 kb). Cc resistance was associated only with pBI136 but pBI106 was 90% homologous to a previously described Cc resistance plasmid (pBF4) with the exception of the specific pBF4 region implicated in the resistance. Restriction endonuclease maps of pBI136 and pBI106 were constructed and the Cc resistance determinant on pBI136 was localized. This region of pBI136 was shown to have homology to the Cc resistance determinant of pBF4. The pBI136 Cc resistance determinant has been cloned and its structure studied. Results indicate a transposon-like structure with the Cc resistance gene flanked by directly repeated sequences of 1.2 kb. Comparative studies with two other Bacteroides strains have been initiated. Transmissible Cc resistance among Bacteroides spp. also occurs by a process which does not involve detectable extrachromosomal elements. The plasmid pBI136 is extremely difficult to detect and may provide insight into the mechanism of the non-plasmid associated transmissible Cc resistance. In collaboration with Dr. Macrina, pBI136 is being used to probe the molecular structure of one such model system in Bacteroides fragilis strain V503.